I am developing a software with java, it creates a thread upon receiving an event (from sensors). the time-to-live of these threads is very small (< 1 second)
The sensor sends maximal 10 events/minute.
This app works fine for most of the time. but sometime it hangs.
When looking at the eclipse debugger, I find out that there are to many threads and most of them are "Thread[garbage collected]" (about 800 threads #_#)
I don't know if that bug is caused by dynamic-creating-threads in my code or other bugs?
EDIT:
The problem is indeed caused by creating too many threads. I have logged all sensor's events with timestamp and there are some points it creates about 1200 events/minute (damn!).
I also wrote a small java program which creates as many threads as posible. At ~4100th thread (well, wooden computer) , jvm crashes. It does not hangs like my app does :-?.
Therefore I suppose that there are (maybe) rare conditions while creating threads dynamically and it causes the garbage collection threads hang?
Don't create a new thread for each event that is received. Instead, use the classes from the java.util.concurrent package.
Create an ExecutorService (using one of the static methods in class Executors) and submit jobs for handling events to the ExecutorService. The ExecutorService is a thread pool that manages the threads for you.
I had a similar problem in NetBeans. After a while, the program would hang with lots (maybe 500) of suspended threads. However, when run outside the debugger it worked just fine. I don't think I ever got more than a couple hundred threads going at once running when running outside, but the program did tend to start them at a furious rate. My suspicion was that the debugger never shut down a thread and could only handle so many.
My experience so far is that the JVM handles vast numbers of rapidly starting and stopping threads very well, but that the NetBeans debugger (several versions ago now, one of the 6's) did not.
Related
If I have a thread that is blocked waiting for a lock, can the OS reschedule that thread to do other work until the lock becomes available?
From my understanding, it cannot be rescheduled, it just sits idle until it can acquire the lock. But it just seems inefficient. If we have 100 tasks submitted to an ExecutorService, and 10 threads in the pool: if one of the threads holds a lock and the other 9 threads are waiting for that lock, then only the thread holding the lock can make progress. I would have imagined that the blocked threads could be temporarily rescheduled to run some of the other submitted tasks.
You said:
I would have imagined that the blocked threads could be temporarily rescheduled to run some of the other submitted tasks.
Project Loom
You happen to be describing the virtual threads (fibers) being developed as part of Project Loom for future versions of Java.
Currently the OpenJDK implementation of Java uses threads from the host OS as Java threads. So the scheduling of those threads is actually controlled by the OS rather than the JVM. And yes, as you describe, on all common OSes when Java code blocks, the code’s thread sits idle.
Project Loom layers virtual threads on top of the “real” platform/kernel threads. Many virtual threads may be mapped to each real thread. Running millions of threads is possible, on common hardware.
With Loom technology, the JVM detects blocking code. That blocked code’s virtual thread is “parked”, set aside, with another virtual thread assigned to that real thread to accomplish some execution time while the parked thread awaits a response. This parking-and-switching is quite rapid with little overhead. Blocking becomes extremely “cheap” under Loom technology.
Blocking is quite common in most run-of-the-mill business-oriented apps. Blocking occurs with file I/O, network I/O, database access, logging, console interactions, GUIs, and so on. Such apps using virtual threads are seeing huge performance gains with the experimental builds of Project Loom. These builds are available now, based on early-access Java 17. The Project Loom team seeks feedback.
Using virtual threads is utterly simple: Switch your choice of executor service.
ExecutorService executorService = Executors.newVirtualThreadExecutor() ;
Caveat: As commented by Michael, the virtual threads managed by the JVM depend on the platform/kernel threads managed by the host OS. Ultimately, execution is scheduled by the OS even under Loom. Virtual threads are useful for those times when a blocked Java thread would otherwise be sitting idle on a CPU core. If the host computer were overburdened, the Java threads might see little execution time, with or without virtual threads.
Virtual threads are not appropriate for tasks that rarely block, that are truly CPU-bound. For example, encoding video. Such tasks should continue using conventional threads.
For more info, see the enlightening presentations and interviews with Ron Pressler of Oracle, or with other members of the Loom team. Look for the most recent, as Loom has evolved.
I would have imagined that the blocked threads could be temporarily rescheduled to run some of the other submitted tasks.
That's what the other threads are for. If you create X threads and Y are blocked, you have the remaining X-Y threads to do other submitted tasks. Presumably, the number X was chosen specifically to get the number of concurrent tasks that the implementation and/or programmer thought was best.
You are asking why the implementation doesn't ignore this decision. The answer is because it makes more sense to choose the number of threads reasonably than have the implementation ignore that choice.
You are partially right.
In the executor service scenario that you described, all the 9 threads will be blocked and only one thread will make progress. true.
The part where you are not quite right is when you attempt to expect the behaviour of OS and Java combined. See, the concept of threads exists at both OS and at Java level. But they are two different things. So there are Java-Threads and there are OS-Threads. Java-Threads are implemented through OS-Threads.
Imagine it this way, JVM has (say) 10 Java-Threads in it, some running, some not. Java borrows some OS-Thread to implement the running Java-Threads. Now when a Java-Thread gets blocked (for whatever reason) then what we know for sure is that the Java-Thread has been blocked. We cannot easily observe what happened to the underlying OS-Thread.
The OS could reclaim the OS-Thread and use it for something else, or it can stay blocked - it depends. But even if the OS-Thread is reused, still Java-Thread will remain blocked. In your thread pool scenario, still the nine Java-Threads will be blocked, and only one Java-Thread will be working.
If I have a thread that is blocked waiting for a lock, can the OS reschedule that thread to do other work until the lock becomes available? From my understanding, it cannot be rescheduled, it just sits idle until it can acquire the lock. But it just seems inefficient.
I think you are thinking about this entirely wrong. Just because 10 of your 20 threads are "idle" doesn't mean that the operating system (or the JVM) is somehow consuming resources trying to manage these idle threads. Although in general we work on our applications to make sure that our threads are as unblocked as possible so we can achieve the highest throughput, there are tons of times that we write threads where we expect them to be idle most of the time.
If we have 100 tasks submitted to an ExecutorService, and 10 threads in the pool: if one of the threads holds a lock and the other 9 threads are waiting for that lock, then only the thread holding the lock can make progress. I would have imagined that the blocked threads could be temporarily rescheduled to run some of the other submitted tasks.
It is not the threads which are rescheduled, it is the CPU resources of the system. If 9 of your 10 threads are blocked in your thread-pool then other threads in your application (garbage collector) or other processes can be given more CPU resources on your server. This switching between jobs is what the modern operating systems are really good at and it happens many, many times a second. This is all quite normal.
Now, if your question is really "how do I improve the throughput of my application" then you are asking the right question. First off, you should make sure that your locks are as fine grained as possible to make sure that the thread holding the lock does so for a minimal amount of time. If the blocking happens too often then you should consider increasing the number of threads in the pool so that there is a higher likelihood that some of the jobs will run concurrently. This optimizing of the number of threads in a thread-pool is very application specific. See my post here for some more details: Concept behind putting wait(),notify() methods in Object class
Another thing you might consider is breaking your jobs up into pieces to separate the pieces that can run concurrently from the ones that need to be synchronized. You could have a pool of 10 threads doing the concurrent work and then a single thread doing the operations that require the locks. This is why the ExecutorCompletionService was written so that something downstream can take the results of a thread-pool and act on them as they complete. This will make your program more complicated and you'll need to worry about your queues if you are talking about some large number of jobs or large number of results but you can dramatically improve throughput if you do this right.
A good example of such refactoring is a situation where you have a processing job that has to write the results to a database. If at the end of each job, each thread in the pool needs to get a lock on the database connection then there will be a lot of contention for the lock and less concurrency. If, instead, the processing was done in a thread-pool and there was a single database update thread, it could turn off auto-commit and make updates from multiple jobs in a row between commits which could dramatically increase throughput. Then again, using multiple database connections managed by a connection pool might be a fine solution.
This is what I see in Oracle documentation and would like to confirm my understanding (source):
A computer system normally has many active processes and threads. This
is true even in systems that only have a single execution core, and
thus only have one thread actually executing at any given moment.
Processing time for a single core is shared among processes and
threads through an OS feature called time slicing.
Does it mean that in a single core machine only one thread can be executed at given moment?
And, does it mean that on multi core machine multiple threads can be executed at given moment?
one thread actually executing at any given moment
Imagine that this is game where 10 people try to sit on 9 chairs in a circle (I think you might know the game) - there isn't enough chairs for every one, but the entire group of people is moving, always. It's just that everyone sits on the chair for some amount of time (very simplified version of time slicing).
Thus multiple processes can run on the same core.
But even if you have multiple processors, it does not mean that a certain thread will run only on that processor during it's entire lifetime. There are tools to achieve that (even in java) and it's called thread affinity, where you would pin a thread only to some processor (this is quite handy in some situations). That thread can be moved (scheduled by the OS) to run on a different core, while running, this is called context switching and for some applications this switching to a different CPU is sometimes un-wanted.
At the same time, of course, multiple threads can run in parallel on different cores.
Does it mean that in a single core machine only one thread can be executed at given moment?
Nope, you can easily have more threads than processors assuming they're not doing CPU-bound work. For example, if you have two threads mostly waiting on IO (either from network or local storage) and another thread consuming the data fetched by the first two threads, you could certainly run that on a machine with a single core and obtain better performance than with a single thread.
And, does it mean that on multi core machine multiple threads can be executed at given moment?
Well yeah you can execute any number of threads on any number of cores, provided that you have enough memory to allocate a stack for each of them. Obviously if each thread makes intensive use of the CPU it will stop being efficient when the number of threads exceeds the number of cores.
When a single user is accessing an application, multiple threads can be used, and they can run parallel if multiple cores are present. If only one processor exists, then threads will run one after another.
When multiple users are accessing an application, how are the threads handled?
I can talk from Java perspective, so your question is "when multiple users are accessing an application, how are the threads handled?".
The answer is it all depends on how you programmed it, if you are using some web/app container they provide thread pool mechanism where you can have more than one threads to server user reuqests, Per user there is one request initiated and which in turn is handled by one thread, so if there are 10 simultaneous users there will be 10 threads to handle the 10 requests simultaneously, now we do have Non-blocking IO now a days where the request processing can be off loaded to other threads so allowing less than 10 threads to handle 10 users.
Now if you want to know how exactly thread scheduling done around CPU core, it again depends on the OS. One thing common though 'thread is the basic unit of allocation to a CPU'. Start with green threads here, and you will understand it better.
The incorrect assuption is
If only one processor exists, then threads will run one after another.
How threads are being executed is up to the runtime environment.
With java there are some definitions that certain parts of your code will not be causing synchronisation with other threads and thus will not cause (potential) rescheduling of threads.
In general, the OS will be in charge of scheduling units-of-execution. In former days mostly such entities have been processes. Now there may by processes and threads (some do scheduling only at thread level). For simplicity let ssume OS is dealing with threads only.
The OS then may allow a thread to run until it reaches a point where it can't continue, e.g. waiting for an I/O operation to cpmplete. This is good for the thread as it can use CPU for max. This is bad for all the other threads that want to get some CPU cycles on their own. (In general there always will be more threads than available CPUs.So, the problem is independent of number of CPUs.) To improve interactive behaviour an OS might use time slices that allow a thread to run for a certain time. After the time slice is expired the thread is forcible removed from the CPU and the OS selects a new thread for being run (could even be the one just interrupted).
This will allow each thread to make some progress (adding some overhead for scheduling). This way, even on a single processor system, threads my (seem) to run in parallel.
So for the OS it is not at all important whether a set of thread is resulting from a single user (or even from a single call to a web application) or has been created by a number of users and web calls.
You need understand about thread scheduler.
In fact, in a single core, CPU divides its time among multiple threads (the process is not exactly sequential). In a multiple core, two (or more) threads can run simultaneously.
Read thread article in wikipedia.
I recommend Tanenbaum's OS book.
Tomcat uses Java multi-threading support to serve http requests.
To serve an http request tomcat starts a thread from the thread pool. Pool is maintained for efficiency as creation of thread is expensive.
Refer to java documentation about concurrency to read more https://docs.oracle.com/javase/tutorial/essential/concurrency/
Please see tomcat thread pool configuration for more information https://tomcat.apache.org/tomcat-8.0-doc/config/executor.html
There are two points to answer to your question : Thread Scheduling & Thread Communication
Thread Scheduling implementation is specific to Operating System. Programmer does not have any control in this regard except setting priority for a Thread.
Thread Communication is driven by program/programmer.
Assume that you have multiple processors and multiple threads. Multiple threads can run in parallel with multiple processors. But how the data is shared and accessed is specific to program.
You can run your threads in parallel Or you can wait for threads to complete the execution before proceeding further (join, invokeAll, CountDownLatch etc.). Programmer has full control over Thread life cycle management.
There is no difference if you have one user or several. Threads work depending the logic of your program. The processor runs every thread for a certain ammount of time and then follows to the next one. The time is very short, so if there are not too much threads (or different processes) working, the user won't notice it. If the processor uses a 20 ms unit, and there are 1000 threads, then every thread will have to wait for two seconds for its next turn. Fortunately, current processors, even with just one core, have two process units which can be used for parallel threads.
In "classic" implementations, all web requests arriving to the same port are first serviced by the same single thread. However as soon as request is received (Socket.accept returns), almost all servers would immediately fork or reuse another thread to complete the request. Some specialized single user servers and also some advanced next generation servers like Netty may not.
The simple (and common) approach would be to pick or reuse a new thread for the whole duration of the single web request (GET, POST, etc). After the request has been served, the thread likely will be reused for another request that may belong to the same or different user.
However it is fully possible to write the custom code for the server that binds and then reuses particular thread to the web request of the logged in user, or IP address. This may be difficult to scale. I think standard simple servers like Tomcat typically do not do this.
I just want to ask a rookie question: How to set an appropriate thread number for my thread pool on the server side?
Are there any general rules or formulas I can follow?
What are the issues I have to consider? For example, the number of network requests per second, the number of CPU cores, the CPU and memory usage rate in my application, the hardware I use on my server, etc.
Well, basically the size of the pool should be set to the the maximum possible of commands executed concurrently on your configuration, like if you have 4 cores (without HyperThreading), then you can set it to 4. With hyperthreading, you can set it to 8.
There are however questions like: what is the expected behaviour of the application, if it wants to get a thread from the pool, but the pool is empty (like you had 8 threads in the pool, every single one if them is working on a video encoding job in the next 10 minutes, and you get a new request in your manager thread).
You should consider however, that it is NOT guaranteed, that all your threads will run in every moment, even if your application handles threading exceptionally perfectly, as other applications are running on your computer meanwhile (your OS for example), and they need CPU as well.
On the other hand it is also a big question, that what does a thread do in your pool? You provided no informations about what is this thread pool used for, are they used in your own app, or you want to configure an open-source app/commercial app, etc. Creating and managing threads do have serious costs (scheduling, context switching, etc.), which may worth only if, the your threads stay alive long enough (you can provide enough job them to work on).
For further details, a quite good starting point in this subject could be Google I guess, for the following keywords: "scheduling, concurrency, threads, java executor service, hyperthreading".
My java desktop application runs every time the computer starts up and runs as long as computer is ON.
It monitors all the activities and interacts with database more often and will tell me some updates by looking on the internet at regular intervals etc..etc..
So what I usuallly do is creating the threads and calling the sleep method. BUt is there any way so that i can handle the memory more efficiently as my program runs 24/7. Are there any methods which will be very much useful if we want to make the program sit silently if there is no job to do . Any advice or suggestion will be helpful
Thanks in advance
Only allocate as much memory as you need, in other words, do not keep stuff that you do not need.
Also, run at lower priority; though with sleep-calls, this won't gain you much.
Of course you could, or depending on the kind of application, should, also quit the program after doing the job, and only let it restart by cron, anacron or similar services (depending on your operating system).
I think you could use a Thread pool for your tasks,
This hold some threads active and when one of your tasks needs a thread you can request one, this way you need less active trheads running
Oracle about thread pools